Sulphide oxidation in leaching of minerals
A process for leaching minerals that contain metal sulphides and one or more precious metals or precious metal compounds, the process comprising the steps of a first leaching step to leach the minerals under oxidative conditions at a pH of less than 4 to form a slurry or pulp, the slurry or pulp comprising a solid phase containing unreacted components, solid reaction products and elemental sulphur, and subjecting the slurry or pulp or solid residue from the first leaching step to a second leaching step comprising oxidative leaching at pH of at least 9.0 to thereby form thiosulphate, whereby the thiosulphate leaches precious metal from the solid residue.
1 . A process for leaching minerals, the minerals containing metal sulphides and one or more precious metals or precious metal compounds, the process comprising the steps of leaching the minerals under oxidative conditions at a pH of less than 4 to form a slurry or pulp, the slurry or pulp comprising a solid phase containing unreacted components, solid reaction products, and elemental sulphur, and subjecting the slurry or pulp to oxidative leaching at pH of at least 9.0 to thereby form thiosulphate, whereby the thiosulphate leaches precious metal from the solid residue.
2 . The process as claimed in claim 1 , further comprising separating a leach liquor from the step of oxidative leaching at pH of at least 9.0 and recovering precious metals from the leach liquor.
3 . The process as claimed in claim 1 , wherein the minerals are subjected to a first leaching step conducted under oxidative conditions at a pH of less than 4, and the solid residue from the first leaching step is subjected to a second leaching step conducted under oxidative conditions and under alkaline conditions, at a pH of greater than 9.0.
4 . The process as claimed in claim 3 , wherein elemental sulphur is formed in the first leaching step and the elemental sulphur is separated from pregnant leach liquor with the solid residue and in the second leaching step, the elemental sulphur reacts with oxygen at the alkaline conditions to form thiosulphate in-situ, and the precious metals are leached into solution in the second leaching step.
5 . The process as claimed in claim 1 , wherein the slurry formed in the first leaching step is sent to the second leaching step without subjecting the slurry to solid/liquid separation.
6 . The process as claimed in claim 5 , wherein an intermediate neutralisation step is performed to increase the pH of the slurry or pulp prior to feeding the slurry or pulp to the second leaching step.
7 . The process as claimed in claim 1 , wherein the leaching liquor from the second leaching step is separated from the solids and dissolved precious metals are recovered therefrom.
8 . The process as claimed in claim 1 , wherein unreacted sulphides that are present in the solid residue fed to the second leaching step are also oxidised in the second leaching step and breakdown of solid reaction products formed in the first leaching step also occurs in the second leaching step, whereby precious metals are leached into solution in the second leaching step and are recovered from the leach liquor, and elemental sulphur that is present is consumed.
9 . The process as claimed in claim 1 , wherein the first leaching step is conducted at a pH of less than 4.
10 . The process as claimed in claim 1 , wherein the minerals that are fed to the first leaching step are finely ground.
11 . The process as claimed in claim 10 , wherein the minerals that are fed to the first leaching step are ground such that they have a P 80 of 20 μm or less.
12 . The process as claimed in claim 1 , wherein the minerals comprise a sulphide mineral composition, ore or concentrate.
13 . The process as claimed in claim 1 , wherein the minerals are selected from one or more of chalcopyrite, bornite, enargite, pyrite, covellite, sphalerite, chalcocite, pentlandite, cobaltite, pyrrhotite or mixtures of any two or more thereof.
14 . The process as claimed in claim 1 , wherein sulphide minerals fed to the first leaching step are subject to fine grinding in a mill and ground to a maximum average particle size of 80% passing size of 20 microns as measured with a laser sizer, or ground to a particle size distribution having a P 80 of 12 micron or less.
15 . The process as claimed in claim 1 , wherein the first leaching step is conducted at atmospheric pressure and at a temperature up to the boiling point of the mixture.
16 . The process as claimed in claim 1 , wherein oxidative leaching conditions are obtained in the first leaching step by sparging with an oxygen-containing gas.
17 . The process as claimed in claim 1 , wherein the mixture of solid residue and pregnant leach solution from the first leaching step is separated using a liquid/solid separation technique, selected from filtration, sedimentation, clarification, and the solid residue is optionally washed with wash water to remove any residual leach liquor therefrom.
18 . The process as claimed in claim 17 , wherein the solid residue is treated in a second leaching step under oxidative conditions, and at a pH of at least 9.0, and during which: (i) unreacted sulphides from the first leaching step are oxidized, and (ii) elemental sulphur in the solid residue is reacted to form thiosulphate.
19 . The process as claimed in claim 1 , wherein a leach liquor containing dissolved precious metal is separated from the solid residue from the second leaching step, and precious metal is recovered from the leach liquor.
20 . A process for leaching minerals, the minerals containing metal sulphides and one or more precious metals or precious metal compounds, the process comprising the steps of leaching the minerals under oxidative conditions at a pH of less than 4 to form a pregnant leach liquor containing dissolved metal and a solid residue containing unreacted components, solid reaction products and elemental sulphur, separating the solid residue from the pregnant leach liquor, and subjecting the solid residue to oxidative leaching at pH of at least 9.0 to thereby form thiosulphate, whereby the thiosulphate leaches precious metal from the solid residue.